Why DMD is so slow?

[janderson]

Marco wrote:
> I have written the code reported below to test execution speed of D in Windows and I have found that the same code is about 10 times slower if compiled using DMD w.r.t. GCD: 1.9 minutes in DMD vs 1.84 seconds in GDC ! Is there perhaps something wrong? why such a difference?
> Thank you.
> 
> // begin of file mandel_d1.d
> /*
> DMD: dmd -inline -release -O mandel_d1.d
> GCD: gdc -O3 --fast-math -inline -lgphobos --expensive-optimizations mandel_d1.d
> */
> 
> import std.stdio;
> 
> int main()
> {
>  cdouble a, b, c, z;
>  double mand_re = 0, mand_im = 0;
> 
>  for (double y = -2; y < 2; y += 0.01) {
>   for (double x = -2; x < 2; x += 0.01) {
>    z = (x + mand_re) + (y + mand_im) * 1i;
>    c = z;
>    for (int i = 0; i < 10000; i++) {
>     z = z * z + c;
>     if(z.re * z.re + z.im * z.im > 4.0) {
>      break;
>     }
>    }
>   }
>  }
>  return 0;
> }
> // end of file mandel_d1.d
> ------------------
> 
> H:\Codici\Benchmarks> ..\timethis.exe mandel_d1
> 
> TimeThis :  Command Line :  mandel_d1
> TimeThis :    Start Time :  Mon Jun 02 11:28:41 2008
> 
> 
> TimeThis :  Command Line :  mandel_d1
> TimeThis :    Start Time :  Mon Jun 02 11:28:41 2008
> TimeThis :      End Time :  Mon Jun 02 11:30:35 2008
> TimeThis :  Elapsed Time :  00:01:54.234
> 
> H:\Codici\Benchmarks> ..\timethis mandel_gdc1
> 
> TimeThis :  Command Line :  mandel_gdc1
> TimeThis :    Start Time :  Mon Jun 02 11:42:27 2008
> 
> 
> TimeThis :  Command Line :  mandel_gdc1
> TimeThis :    Start Time :  Mon Jun 02 11:42:27 2008
> TimeThis :      End Time :  Mon Jun 02 11:42:29 2008
> TimeThis :  Elapsed Time :  00:00:01.843
> 

It would be interesting to compare the ASM produced.  DMD is not that 
great at floating point, doesn't unroll so well and has a longer startup 
time then GDC in my experience.

-Joel